Recently, since an optical disk is capable of recording a large quantity of information at a high density, the utilization of the optical disks has been promoted in various fields such as audiovisual apparatuses, videos, and computers. In this trend, an increase in the recording capacity has been further demanded, on which various attempts have been made, including a method of providing an optical disk having a multilayer configuration.
For example, the Japanese Publication of Laid-Open Patent Application No. 5-151609/1993 (Tokukaihei 5-151609) discloses an optical disk apparatus for individually reproduce recording data from each data layer of an optical disk which has a multiple data layer configuration. Operational principles of this optical disk is described below, with reference to FIG. 18.
Recording/reproducing layers 31 through 38 of a multilayer disk 30 are formed between transparent substrates 21 and air layers 22 which are alternately provided. During an information recording and reproducing operation, a laser beam is focused on each of the recording/reproducing layers 31 through 38, by moving an objective lens 4 in an optical axis direction with the use of an actuator 23. In the case where, during the focusing, a focal point of the laser beam is being moved in the vicinity of one recording/reproducing layer in the optical axis direction with the move of the objective lens 4, a focus error signal (hereinafter referred to as FES) has a sigmoid waveform (hereinafter referred to as FES curve), becoming 0 when the laser beam is rightly focused.
FIG. 19 illustrates an FES curve in this case. Since the recording/reproducing layers in this case have intervals therebetween of around 400 .mu.m each which is sufficient, no reverse affection is caused by return lights from adjacent recording/reproducing layers. Therefore, when the objective lens 4 is moved, substantially independent FES curves are obtained with respect to each of the recording/reproducing layers 31 through 38. Specifically, since the layers are provided sufficiently apart from each other, an FES curve of an n'th layer becomes 0 when an n+1'th layer or an n-1'th layer is focused, thereby causing no affection such as offsets to FES curves of the other layers.
Note that in this case, since each interval between the layers is not small, a thickness of the disk substrate to be focused changes to a great extent, when a focus servo is carried out with respect to each layer. Therefore, as illustrated in FIG. 18, it is necessary to individually correct spherical aberrations caused in each plane with the use of an aberration compensator 26.
On the other hand, a disk of a two-layer structure, wherein data layers are provided at a sufficiently small interval compared with a thickness of a substrate, for example, 30 to 40 .mu.m, has been proposed as a digital video disk (DVD) or the like. In this case, since spherical aberrations due to differences between thicknesses of individual substrates are sufficiently small, the aberration compensator 26 is unnecessary. As an optical system suitable for such a disk of the two-layer structure wherein data layers are provided at a sufficiently small interval, an optical system disclosed in the Japanese Publication for Laid-Open Patent Application No. 4-21928/1992 (Tokukaihei 4-21928) is given as an example.
FIG. 20 illustrates the optical system disclosed in the above publication. A light emitted from a semiconductor laser 1 as a light source is diffracted by a hologram element 2, and a zeroth diffracted light, among all the lights obtained by diffraction, enters a reshaping prism 24 through a collimator lens 3. A light beam, reshaped by the reshaping prism 24, is converged on an optical disk 6 through an objective lens 4. A return light of the light beam passes through the reshaping prism 24 and the collimator lens 3 and is directed to the hologram element 2, thereby being caused to form a beam R whose cross section is oval as illustrated in FIG. 21. The beam R is projected on a light receiving element 25 (see FIG. 22), and the light receiving element 25 converts optical signals obtained from the beam R into electric signals, and outputs the electric signals.
When being viewed from the side of the optical disk 6, the hologram element 2 is, as illustrated in FIG. 21, divided into three division regions 2a, 2b, and 2c, by a division line 2g in a y direction which is conformed to a radial direction of the optical disk 6, and a division line 2h which starts at the midpoint of the division line 2g and is directed in an x direction which is orthogonal to the radial direction of the optical disk 6, that is, directed in a track direction of the optical disk 6. So as to correspond to each of the division regions 2a through 2c, respective gratings are prepared.
As shown in FIGS. 22(a) through 22(c), the light receiving element 25 has four rectangular light receiving regions 25a through 25d which are lined in the x direction which conforms to the track direction of the optical disk 6. The light receiving regions 25a and 25b (focusing-use light receiving regions) are provided adjacent in the center with a division line 25x therebetween, which is directed in the y direction which conforms to the radial direction of the optical disk 6. On the other hand, the light receiving regions 25c and 25d (tracking-use light receiving regions) are respectively provided beside the light receiving regions 25a and 25b with a predetermined interval each therebetween, so that the light receiving regions 25a through 25d are lined in the x direction. Each of the light receiving regions 25a through 25d has a long rectangular shape with its longitudinal direction conformed to the y direction which corresponds to the radial direction.
When the light from the semiconductor laser 1 is focused with respect to the optical disk 6, a focusing-use return light, which has been diffracted at the division region 2a of the hologram element 2, forms a dot-like beam spot P1 on the division line 25x, as illustrated in FIG. 22(a). A tracking-use return light which has been diffracted at the division region 2b forms a beam spot P2 on the light receiving region 25d, while a tracking-use return light which has been diffracted at the division region 2c forms a beam spot P3 on the light receiving region 25c. The beam spots P1 through P3 may in some cases be formed at positions somewhat displaced in the y direction from the respective centers of the light receiving regions 25a through 25d, so that position tolerance of the light receiving element 25, drift of a wave length of the light, or the like, are absorbed by adjusting the position of the hologram element 2.
The beam spot P1 expands either in the light receiving region 25b or in the light receiving region 25a, as shown in FIG. 22(b) in the case a distance between the optical disk 6 and the objective lens is too great, and in FIG. 22(c) in the case the distance is too small. Here, when output signals from the light receiving regions 25a, 25b, 25c, and 25d are given as Sa, Sb, Sc, and Sd, respectively, the focusing error signal FES can be obtained by calculation of (Sa-Sb) by the single knife edge method.
A tracking error signal (hereinafter referred to as RES) can be obtained by calculation of (Sc-Sd) by the push-pull method, namely, by comparing diffracted lights respectively from the division regions 2b and 2c which are thus divided by the division line 2h directed in the track direction of the optical disk 6.
The following description will discuss in detail the correlation between the FES curve and the beam spot formed on the light receiving element 25. The hologram element 2 and the light receiving element 25 are adjusted so that in a focalizing state the beam spot P1 is formed on the division line 25x as shown in FIG. 22(a), namely, so that the light is evenly projected on the light receiving regions 25a and 25b. When the objective lens 4 is positioned farther from the optical disk 6 than when it is in the focalizing state, the beam spot P1 expands in the light receiving region 25b as shown in FIG. 22(b). Therefore, a light quantity in the light receiving region 25b increases, thereby causing the FES to have a negative value. In contrast, when the objective lens 4 is positioned closer, the beam spot Pi expands in the light receiving region 25a as shown in FIG. 22(c), and a light quantity of the light receiving region 25a increases, thereby causing the FES to have a positive value.
The FES curve obtained in this case is a curve F' shown in FIG. 23, which substantially linearly changes till the beam spot P1 protrudes from the light receiving region 25a or 25b. When the beam spot P1 is protruding from the light receiving region 25a or 25b, the light quantity in the light receiving region 25a or 25b is decreasing, thereby causing the absolute value of the FES to decrease and finally converges to 0.
A range between (1) the position of the objective lens 4 when the beam spot P1 starts protruding from the light receiving region 25a in the case where the objective lens 4 becomes closer to the optical disk 6 than when it is in the focalizing state, and (2) the position of the objective lens 4 when the beam spot P1 starts protruding from the light receiving region 25b in the case where the objective lens 4 becomes farther, is called a dynamic range Dy. In other words, a distance Dy between the positive peak of the FES and the negative peak of the FES is called the dynamic range Dy. Usually a width of about 15 .mu.m is required for the dynamic range Dy, so that a pull-in range of the focus servo is ensured. The respective widths of the light receiving regions 25a and 25b are determined so that within the dynamic range Dy, the beam spot P1 does not protrude from the light receiving regions 25a and 25b. With greater widths of the light receiving regions 25a and 25b, the dynamic range Dy can be set wider, but on the contrary the light receiving regions have greater areas thereby causing frequency characteristics to deteriorate. Therefore, the dynamic range Dy is set to the narrowest possible.
Note that in the following descriptions, a state wherein the objective lens 4 is displaced from the position in the focalizing state outside the Dynamic range Dy, that is, greatly close to or greatly far from a recording/reproducing layer of the optical disk 6 to be focused, is regarded as a greatly defocusing state. On the other hand, a state wherein the objective lens 4 is displaced within the Dynamic range Dy, that is, slightly close or slightly far from a recording/reproducing layer to be focused, is regarded as a slightly defocusing state.
However, in the case where reproduction is carried out with the use of an optical system shown in FIG. 20 with respect to a multilayer disk wherein the layers are provided at just small intervals (d1 in FIG. 23) of about twice of the width of the dynamic range Dy each, for example, offsets occur to the FES, since one FES curve which has been obtained with respect to a data surface of a layer does not sufficiently converge to 0 when another FES curve is obtained with respect to a data surface of an adjacent layer.
FIGS. 24 and 25 illustrate FES curves obtained from two layers (first and second layers) adjacent to each other in a plurality of layers. The horizontal axis T represents displacement of the objective lens 4, and T1 and T2 represent respective positions of the objective lens 4 when the convergent beam is rightly focused on the first and second layers. An FES curve obtained with respect to the first layer is denoted F1', while an FES curve obtained with respect to the second layer is denoted F2'. But, provided that respective quantities of light reflected from the first and second layers are equal to each other, what is actually obtained is an FES curve indicated by a broken line denoted F3' in FIG. 24, which is a resultant curve of the curves F1' and F2'.
A solid line in FIG. 25 indicates the FES curve F3'. As is clear from FIG. 25, since the FES curves obtained with respect to the data surface of the adjacent layer does not converge to 0, FES offsets .DELTA.f1 and .DELTA.f2 respectively occur at T1 and T2 which are the positions of the objective lens 4 when the convergent beam is rightly focused on the first and second layers, respectively. Therefore, detection sensitivity also changes.
FIGS. 24 and 25 illustrate only FES curves obtained with respect to the two adjacent layers, but in the case with three or more layers, such reverse affect is multiplied, thereby causing such offsets and detection sensitivity to further change, and hence making it more difficult to conduct a normal focus servo.